Two men undertake to do a work for Rs. 200. One alone could do it in 6 days the other in 8 days, With the assistance of a boy they finish it in 3 days. How much money should be given to the boy?

First man's rate: 1/6 work per day. Second man's rate: 1/8 work per day. Together with boy, they finish in 3 days, so combined rate is 1/3 per day. Boy's rate = 1/3 - (1/6 + 1/8) = 1/3 - 7/24 = 1/24. Boy does 3 × (1/24) = 1/8 of work. His share = (1/8) × Rs. 200 = Rs. 25. Options B (Rs. 20), C (Rs. 45), and D (Rs. 65) are incorrect.